Ethtool and mii-tool are tools to set network interface options. I had problems setting speed 1000 with mii-tool so I prefer ethtool.
ethtool -s eth0 speed 100 duplex full autoneg off
If you want to set this every reboot put above command /etc/rc.local or
add following option to this file /etc/sysconfig/network-scripts/ifcfg-eth0
ETHTOOL_OPTS="speed 100 duplex full autoneg off"